1. Field of the Invention
This invention relates to a pneumatic tire having excellent all-over running performances such as traction and braking performances, cornering property and straight running stability and the like without sacrificing the other tire properties.
2. Description of Related Art
As a vehicle performs various behaviors such as straight running, cornering, deceleration and/or stop and the like, longitudinal force, lateral force and so on are applied to a tire from a road surface through a ground contact region, so that the tire is naturally required to have a rigidity durable to these forces. Also, the tire must have a sufficient gripping force against the road surface. By enhancing these forces all-over running performances such as traction and braking performances, cornering property, the straight running stability and the like can be improved.
In general, the gripping force of the tire against the road surface corresponds to friction force between the ground contact region of the tire and the road surface, so that the enhancement of the friction force makes the enhancement of the gripping force and hence improves the above running performances.
As a technique for enhancing the friction force, it is useful to conduct the improvement of a tread portion, concretely the improvement of a distribution of grooves (for example, circumferential grooves, lateral grooves, etc.) arranged in the tread portion, a shape of each groove, a shape of a land portion and the like.
However, the improvement of the tread portion (particularly, a tread pattern) has a large influence on drainage property, noise level, ride comfortability against vibrations, wear resistance and the like, so that it is most difficult to improve the running performance by the conventionally practiced improvement of the tread portion while maintaining the above properties.
Now, the inventor has made detailed investigations with respect to a state of contacting the tread portion with the road surface (concretely, a distribution of ground contact pressure) by using the conventional tire having a usual block pattern in order to improve the friction force between the ground contact region of the tire and the road surface.
As a result, it has been confirmed that when block-shaped lands located in different regions of the tread are compared with each other, the ground contact pressure of the block-shaped land located in each side region of the tread is higher than that of the block-shaped land located in a central region thereof, and in case of viewing only a block-shaped land, a contact pressure (ground contact pressure) at an edge portion of the block-shaped land is higher than that at the other portion thereof and particularly the concentration of ground contact pressure in the edge portion (block edge) becomes remarkable when a large friction force is generated.
From these facts, it has been confirmed that although the conventional tires have a difference in the ground contact pressure or the ground contact area, it tends to produce a portion of the block-shaped land insufficiently contacting with ground by applying external forces such as longitudinal force, lateral force and the like from different directions to the tire. The ground contact area of the tread portion as a whole for generating the above friction force is apt to be decreased and hence the sufficient friction force can not be generated.
Also, it has been confirmed that the tendency of concentrating the ground contact pressure in the edge portion of the block-shaped land becomes conspicuous when the edge portion is in a square state, i.e., at an initial wearing stage of the tire.
From the above, the inventor has thought that the friction force generated between the ground contact region of the tire and the road surface can effectively be enhanced by mitigating the concentration of the ground contact pressure in the edge portion of the block-shaped land to uniformize the ground contact pressure in each of the block-shaped lands.
Moreover, it is useful to chamfer the edge portion of the block-shaped land as means for uniformizing the ground contact pressure in the block-shaped land. As such a tire, there are known, for example, a tire wherein a curved or flat chamfered portion is formed in only edge portions of the block-shaped land corresponding to stepping-in side and kicking-out side of the tire (JP-A-64-22601 etc.) and a tire wherein all edge portions of the block-shaped land is chamfered at a uniform inclination angle over a full periphery thereof (JP-A-2-179508).
However, the former tire has an effect of mitigating the ground contact pressure in the edge portions of the block-shaped land corresponding to the stepping-in side and kicking-out side of the tire. However, when the lateral force and the like are applied to the tire, it can not sufficiently be mitigated to concentrate the ground contact pressure in an edge portion of the block-shaped land facing a circumferential main groove.
In the latter tire, all edge portions of the block-shaped land are chamfered at a uniform inclination angle over its full periphery, so that there is an effect of mitigating the concentration of the ground contact pressure in the edge portions of the block-shaped land as compared with a tire wherein all edge portions of the block-shaped land are not subjected to chamfering. However, since uniform chamfering is carried out over the full periphery of the block-shaped land, the ground contact area of the block-shaped land is largely decreased and hence the friction force can not effectively be generated likewise the former case.